1. Field of the Invention
The present invention relates to a data storage device used for data transmission through a bus, and also relates to a data transmission system that operates with such a data storage device.
2. Description of the Related Art
Data transmission through one or more buses on a single circuit board or in a single system is known in the art.
In order to transmit data between a processing circuit (e.g., micro-controller) and an external memory (e.g., random access memory), the processing circuit and external memory are connected to each other by data and address buses. The processing circuit and external memory are also connected to each other by read and write control signal lines.
The processing circuit outputs a read address on the address bus to read data from the external memory. Upon receiving the read address via the address bus, the external memory locates data in its storage on the basis of the read address and outputs the data on the data bus. The processing circuit then makes the read control signal active, and takes the data from the data bus. The read control signal informs the external memory of a fact that the data has been read.
When the processing circuit writes data into the external memory, on the other hand, the processing circuit outputs a write address on the address bus and a write data on the data bus, and turns the write control signal to an active state. Upon detecting the active write control signal, the external memory receives the address from the address bus and the data from the data bus, and writes the data into a storage area specified by the address.
Other types of processing circuits are also known in the art. For example, a certain type of processing circuit does not write data into an external memory. In other words, the processing circuit is designed to only read data from the external memory. This is because data writing is not performed when the external memory is a ROM (Read Only Memory) or the like.
If the processing circuit does not write data into an external circuit (or memory), the processing circuit naturally does not have a function of generating and outputting a write control signal. An interface for this type of processing circuit can be coupled to the address bus, data bus and read control signal line, but cannot be coupled to the write control signal line.
A system equipped with a processing circuit such as a micro-controller often increases functions by replacing an external circuit or attaching an additional external circuit. Such function expansion sometimes entails modification of the system; if the processing circuit is originally designed not to write data into the external memory, a user of the modified system may want the processing circuit to be able to perform the data writing.
Since the processing circuit designed to perform no data writing to an external circuit (memory) does not have a function of producing and outputting a write control signal, such processing circuit is no use in the system in terms of data writing from the processing circuit into an external memory. In order for the system to be able to write data into the external memory, therefore, the processing circuit should be replaced with another processing circuit. The processing circuit is, however, very expensive.
There is a demand for a technique that operates or controls a processing circuit, which is designed not to write data into an external circuit, to be able to write data into an external memory or the like.